Gasification-pyrolysis dual reactor device

ABSTRACT

The device herein described includes: a feeder for feeding wet crushed coal from an upper feed nozzle ( 1 ) into two branches ( 1   a  and  1   b ) provided with suitable mills; an element for mixing/distributing screened coal ( 2 ), located downstream of the nozzle ( 1 ); a gasification chamber ( 3 ) located downstream of the aforementioned element ( 2 ) where the screened material is oxidised by means of the supply of oxygen at approximately 1,800° C.-1,900° C., said chamber ( 3 ) being divided into two sub-chambers by a gas diffusion membrane; a cyclone separator ( 4 ) located downstream of the chamber ( 3 ), which retains the solid particles present in the synthesis gas; an essentially pyrolytic chamber ( 5 ) where the solids carried from the cyclone separator ( 4 ) are pyrolysed, the residual gases being fed back into the chamber ( 3 ); and a sawtooth screen ( 6 ) which collects the solid waste, storing same as slag in a storage element ( 7 ).

The invention relates to a gasification-pyrolysis dual reactor deviceespecially intended to be used in a gasification-pyrolysis procedure,which is fed with a crushed coal mixture to obtain a synthesis gas,which essentially consists of CO, CO₂ and H₂, using the solar energyobtained at a high-temperature solar thermoelectric plant as steam andoxygen.

Basically, the coal gasification and pyrolysis reactions occur withinthe dual reactor device of the invention, such coal being crushed at thedevice to obtain a synthesis gas. The steam originating from a solarhigh-temperature thermal power plant is fed to thegasification-pyrolysis dual reactor device which has already been fedwith crushed coal, which is partially oxidized with O₂, preferablyobtained at an electrolysis stage, and with the steam coming from thesolar plant, at a gasification chamber provided inside the dual reactordevice. The gas thus obtained basically consists of H₂, CO, with smallamounts of CO₂, CH₄, H₂S and free carbon. To eliminate them, and for theconditioning of the synthesis gas, a pyrolysis process is performed atthe dual reactor device itself, to essentially eliminate the freecarbon, the H₂S and part of CO₂. Briefly explained, the devolatilizationprocess and the release of carbon and hydrogen occur between 38 and 705°C., while C y H₂O(g) are released at a temperature ranging from 705 to1480° C. and C and O₂ between 1480 and 1815° C.

CH₄+CO+CO₂+O₂→CO+CO₂+H₂

The dual reactor device according to the invention comprises thenecessary catalysers to allow the occurrence of the different reactions,as well as the necessary means to maintain its integrity, such asrefractory cladding, insulating materials, refrigerating sleeves, etc.On the other hand, the dual reactor device according to the inventionincorporates a cyclone separator to eliminate any solid waste of thesynthesis gas obtained. insulating materials, refrigerating sleeves,etc. On the other hand, the dual reactor device of the inventionincorporates a cyclone separator to eliminate any solid waste from thesynthesis gas obtained.

Similarly, the dual reactor device of the invention allows to collectthe residues obtained with the process previously described, that arestored for their subsequent reuse in other processes, for instance,those processes where ashes are used, or for their recycling asfertilisers.

The gasification of a solid product is a thermo-chemical process whichencompasses the thermal decomposition of the organic matter and theaction of a gas which mainly reacts with the carbon residue obtainedfrom such thermal decomposition.

The generic term “gasification” encompasses a large variety ofprocesses, where a broad range of different products can be obtained. Onthe basis of the gasifier agent used, a first classification of thegasification process can thus be obtained:

-   -   With air: the partial combustion with air leads to an exothermic        reaction which produces a gas with a low heating power, which        can be used for energy-related purposes.    -   With oxygen: a gas with an average heating power is obtained,        although it is of a better quality, since it is not diluted with        N2. It can be used, apart from the energy-related applications,        as a synthesis gas to obtain methanol.    -   With steam and/or oxygen (or air): a H2- and CO-rich has is        obtained, which can be used as a synthesis gas to obtain        different compounds (ammonia, methanol, gasolines, etc).    -   With hydrogen: A gas with a high energy content is obtained,        which can be used as a substitute for natural gas, since it has        a large percentage of methane.

Another criterion used to obtain an interesting classification of thegasification processes is based on the relative movement of the gasifieragent and the gasified solid agent inside the gasification chamber. Inaccordance with this criterion, the main types of gasifiers are:downdraft mobile bed gasifiers (also known as fixed bed gasifiers) orupdraft gasifiers, and fluidized bed gasifiers. Other types ofgasifiers, that are used to a lesser extent, are rotary furnaces,cyclone reactors, circulating fluidized bed and dragging gasificationsystems, etc.

A large number of reactions occur during the gasification process, andthe relative importance of such reactions depends on the operatingconditions and the gasifier agent used; however, they can be broadlydivided into the three blocks or stages usually comprised by agasification process:

-   -   Pyrolysis or thermal decomposition where, by means of heating,        the original solid is decomposed into a mixture of solid, liquid        and gas. The solid material obtained at this stage is usually        named char and the liquid residues, owing to the majority        presence of tars and condensable vapours, are known as tar.    -   Oxidation or combustion. It happens when the gasifier agent is        an oxidising product, such as oxygen or air, and involves the        whole series of oxidizing reactions, both of an homogeneous and        of an heterogeneous type, which are mainly of an exothermal        nature, by which the heat necessary for the maintenance of the        process is generated.    -   Reduction or gasification. This stage comprises the solid-gas or        gas phase reactions, by which the solid residue becomes gas.        These are basically endothermic reactions, and some of them        occur to a lesser extent, or they occur only under certain        conditions, as it is the case with some hydrogenation and/or        reforming reactions.

The oxidation and reduction stages can be jointly considered as a singlegasification stage, where all kinds of possible reactions between thechar and the gaseous mixture present may occur.

There are several factors that may influence the gasification process,and a distinction should be made between those which refer to theoperating mode and the gasified solid, and those which are inherent tothe design of the gasification chamber and the ancillary equipment.

Temperature is a significant factor at all stages and, consequently, forthe final performance of the process. Specifically, the proportionsbetween char, tar and gas in the pyrolysis products are closely relatedto the heating speed and the final temperature reached. As a generalidea, it can be stated that, at high heating speeds and when the finaltemperature is high, the product basically obtained is a gas, while atlesser heating speeds and lower final temperatures, liquid or solidproducts are mainly obtained. As far as fluidized bed gasifiers areconcerned, the heating speeds normally obtained are high (up tothousands of EC/s), while in the case of mobile bed reactors, theheating speeds obtained are usually moderate (within the of 0.2-0.5 EC/srange).

Strictly speaking, during the gasification stage, and considering thereversible nature of most reactions, temperature influences the reactionbalances. In general terms, and for different fuels, it can be affirmedthat a temperature increase favours the increase in the content of H₂and CO in the gas obtained, to the detriment of CH₄ and H₂O.

In general terms, an increase in pressure represents a disadvantage forgasification reactions, as the proportion of hydrocarbons and tars isalso increased. Mobile bed gasifiers usually operate at atmosphericpressure, while the fluidized bed units use to operate under pressure,reaching 30 bar in some cases.

The gasification agent/residue ratio is one of the most importantparameters of the gasification process, especially in case that theprocess is self-provisioned by means of a partial oxidation, either withair or with oxygen, of the treated residue. When the values of thisparameter are extremely low, the sufficient amount of energy to keep theprocess operating under appropriate conditions may not be generated,thus leading to a decrease in the performance. On the other hand, whenair is used as the gasification agent, there is also a dilution effecton the part of the N₂. Therefore, each process includes an optimum valueof the gasification agent/residue ratio, which is basically dependant onthe composition of the gasified residue. Thus, for instance, in the caseof forest biomass, it has been noticed that the optimum air/biomassrelationship by weight ranges between 0.5 and 1.6 for the fluidized bedgasifiers, and around 1.5 in the case of mobile bed gasifiers.

Thus, it is possible to offset, on the one hand, the increase intemperature, which entails a decrease in the ratio of condensable andsolid residues generated during the pyrolysis stage and, on the otherhand, the poorer quality of the gas obtained.

On the other hand, the ash content provides information about the amountof solids that must be removed from the gasification reactor per unit ofmass processed. Although residues with up to a 24% of ash content havebeen gasified, it is not advisable to exceed a level of 10%. This ashcontent must be removed from the gasification reactor, to prevent theiraccumulation. In the case of mobile bed gasification reactors, they areextracted from the bottom, and in the case of fluidized bed reactors,the gas speed must guarantee the dragging of the ashes. One of the mostsalient properties of the ashes, in this respect, is their meltingpoint. In case that it is exceeded, slag could be generated, which couldobstruct the equipment. Furthermore, as the ashes are of an inertnature, they do not participate in the chemical balances of thegasification reactions, but they may exercise a catalytic effect,accelerating the gasification reaction of the carbonaceous residue withsteam, especially when there are metal oxides like K₂O, CaO, MgO, P₂O₅,etc.

Similarly, it is necessary to take into account that the processinvolves the heating of particles, that reagents will be spread towardsthe particle and that products will be spread from such particle towardsthe gaseous medium which surrounds it, and also that solid-gas reactionsoccur on the solid surface. Therefore, the particle size influences theamount of time necessary to allow the process to occur and the reactorvolume which is appropriate to that effect. In the case of fluid beds,this parameter also affects the minimum fluidisation speed. To modifythe particle size, solutions like densification and milling could beconsidered, but these possibilities also involve additional expenses. Onthe other hand, scarcely dense residues may generate problems related tothe formation of preferential channels or make the fluidisation moredifficult. In the case of fixed beds, load loss problems may occur, aswell as the collapse of the movement of the bed, depending on the shapeof the particles.

The reactions involved in the gasification of coal are:

C + ½ O₂ →CO ΔHr = −9.25 MJ/Kg C + CO₂ →2CO ΔHr = 14.37 MJ/Kg C + H₂O→CO + H₂ ΔHr = 10.94 MJ/Kg CH₄ + ½ O₂ →CO + 2H₂ ΔHr = −35.7 MJ/kmol H₂ +½ O₂ →H₂O ΔHr = −242 MJ/kmol CO + ½ O₂ →CO₂ ΔHr = −283 MJ/kmol CH₄ + H₂O→CO + 3H₂ ΔHr = 206 MJ/kmol CO + H₂O →CO₂ + H₂ ΔHr = −41.1 MJ/kmol

Among the main types of gasification reactors, we could mention those ofthe fluidized bed type, where the solid is suspended in gas. In thiscase, there are no more or less differentiated areas where the differentprocesses, namely drying, pyrolysis, etc. are performed. Each particleis instantaneously/consecutively/simultaneously subject to theseprocesses at any point of the gasification reactor, once it has beenintroduced in such reactor, and the ashes are finally elutriated by theemerging gas. These characteristics of a high reaction temperature,apart from an excellent mixing capability, make that the temperature andconversion profiles are uniform throughout the reactor, allowing aprecise control of the operating conditions. This is also the underlyingreason of the specific capability (Kg of solid/m³ of reactor) comparedwith other types of reactors.

Even though other types of gasification reactors are used, the currenttrend tends to reunite the pyrolysis and oxidation areas within a singlespace, so that all the pyrolysis products are simultaneously producedand burnt, while the reduction takes place at a lower area, in the samemanner as the traditional downdraft process.

Thus, the object of this invention is to provide agasification-pyrolysis dual reactor device which combines the advantagesof the different types of reactors presently known, obtaining asynthesis gas which is essentially free from tars and which allows towork with solid initial materials with a high moisture content, withoutthe need for the expensive prior drying process and without jeopardizingthe performance of the final process, using coal for that purpose,wherein the reagents necessary for the different processes and theenergy required, in the form of steam, are obtained from a hightemperature solar power plant. To that effect, the reactor device of theinvention allows to maintain the optimum temperature, heating speed andresidence time without encountering the problems that are usuallyrelated to this type of systems, such as the deposition of slag at thelower elements of the system, the loss of volatile matters and theprocesses related to the pulverization and drying of the initialmaterial.

Document EP 98949009.9 relates to a procedure and device to obtain fuelgas, synthesis gas and reduction gas from renewable and fossil fuels,other biomasses, litter or sludge, through their combustion at a burner,adding oxygen gas and/or gases containing oxygen at substoichiometricproportions exceeding the melting temperature of the inorganicfractions, to obtain a gasification medium containing CO₂ and H₂O, wherethe fuel and/or gas are rotated upon their entrance in the combustionchamber, and the liquid mineral components obtained are projectedagainst the burner wall arranged in a substantially vertical position,and such components are separated from the gasification medium thusobtained; the gasification medium is routed towards a gasificationreactor through a central opening made on the bottom of the combustionchamber, where this medium forms an immersion jet; the separated liquidcomponents are evacuated through the central opening of the bottom ofthe combustion chamber, and they are dragged along by the immersion jetof the gasification medium as slag droplets, to be evacuatedsimultaneously with the gas and accelerated against the bottom of thereactor, where they accumulate until they are removed; the gasificationmedium is fed with fuel dust containing coal at the gasificationreactor, and during the development of the gasification reaction, thecarbon dioxide is reduced to obtain carbon monoxide and steam; theimmersion gas jet is deflected above the bottom of the reactor, and thegasification gas obtained at the upper part of the reactor is evacuated,to be subsequently transformed, after the appropriate dusting off andchemical purification, in fuel gas, synthesis gas or reduction gas.

Document EP 92121724.6 describes a set of facilities for the productionof synthesis gas from a carbonous material, comprising a system toobtain a high concentration of solar energy at an elongated,high-temperature focal area, a solar gasification reactor with upper andlower regions, containing, at least, a transparent portion which maytransmit a highly concentrated solar radiation; means to allow theprojection of such elongated, high temperature focal area within thereactor, nozzle means located at the upper region of the reactor; meansfor the injection of a dispersion of a specific carbonous materialwithin the reactor, through the said nozzle means; and evacuation meansfor the removal of the synthesis gas product from the upper region ofthe reactor.

The object of the invention will be now more particularly described withreference to FIG. 1 attached, which shows an embodiment of this dualreactor device.

Generally speaking, the gasification and pyrolysis reactions undergoneby the wet milled coal within the dual reactor device are initiatedthrough the feeding to the device of steam coming from a hightemperature solar thermal power plant, such steam being fed through thegasification/pyrolysis dual reactor through the appropriate nozzles.Before the feeding of steam, the reactor device is fed with wet crushedcoal. To that effect, an upper feeding nozzle (1) has been provided,split in two branches, (1 a) and (1 b) and inside which the crushedcarbon is screened to obtain an appropriate particle size of 90 mesh and490 mesh respectively at each one of the branches (1 a, 1 b), preferablyby means of a jaw crusher and ball mill with a 90 mesh screening at oneof the branches and a ball mill with a 490 mesh screening at the otherone. The material resulting from the screening is collected on ascreened coal mixing/distribution item (2) located downstream thefeeding and milling nozzles (1), and the screened coal falls inside themixing/distribution item (2) both as a result of the fact that part ofthe steam fed has forced a downwards circulation of the coal dust anddue to the weight of the dust itself, and such circulation of gasfosters the flow of coal particles, minimising their deposition at thewalls of the mixing/distribution item (2). The above-mentioned mills, aswell as the mixer, are supplied with the energy required for theiroperation, such energy being preferably obtained from a high temperaturesolar power plant.

While the downwards steam coming from the item (2) maintains this flowof coal particles at an approximate temperature of 220-250° C., andsupplying additional steam coming from the solar plant, the screened andwet initial material enters into the chamber (3), which is essentially agasification chamber, where such initial material is partially oxidisedthrough an additional supply of oxygen, preferably coming from anelectrolysis device associated to the solar power plant. The interior ofthis chamber (3) is maintained at a temperature of approximately 1,800°C.-1,900° C. thanks to the high temperature of the steam fed from thesolar plant. The interior of the chamber (3) is provided with arefractory cladding, to maintain the inner temperature, as well as aninsulating cladding on the outer part, to minimise heat losses caused byradiation to the exterior surface of the walls. In a preferredembodiment of the invention, the refractory material of the chamber (3)is based on an alumina, silica and calcareous fibre cement, preferablyof the Superboard® type, maintaining the insulation, with dimensionalstability, up to 3,500° C.

The chamber (3) is divided into two sub-chambers (3 a, 3 b) by means ofa gas-diffusion membrane which allows to keep separated the H₂-based andthe carbon-based (CO₂, CO) gases obtained as a mixture of synthesis gas

CH₄+CO+CO₂+O₂CO+CO₂+H₂

so that the hydrogen is re-circulated from the reactor towards theexterior, to be used for other chemical processing operations or to bedirectly used as fuel.

Downstream of such chamber (3), a cyclone separator (4) retains thesolid particles present at the synthesis gas and, operating at atemperature of 600° C., which is maintained by the gas inside the unit,drags the solid matter towards an essentially pyrolitic chamber (5). Thesmall amounts of CH₄, H₂S and free carbon that may remain as residues inthe synthesis gas are subject to a pyrolysis treatment inside thechamber (5) of that same dual reactor, essentially eliminating freecarbon, H₂S and part of CO₂. Briefly speaking, the devolatilisation andthe release of carbon and hydrogen occurs between 38 and 705° C., in thecase of C and H₂O(g) the release occurs between 705 and 1480° C., and inthe case of C and O₂, between 1480 and 1815° C. Residual gases are fedback to the chamber (3).

Subsequently, the solid remains of the simultaneous gasification andpyrolysis processes cross a sawtooth screen (6) and are collected asslag and ashes inside an storage element (7) for their subsequent reuse,for instance in processes to obtain concrete, where ashes are used, orfor their recycling as fertilisers.

The interior of the chamber (3) houses the appropriate catalysers thatallow the occurrence of the different reactions, as well as the meansrequired to maintain their integrity, as refractory cladding, insulatingmaterials, refrigerating sleeves, etc. This is also applicable to thechamber (5).

The dual reactor device according to the invention allows therecirculation obtained from one chamber to another of the intermediateresidual gases, so that the flow or dragged flow regime allows thecontinuous feeding of coal, and to easily extract the slag from all thechambers, reducing the synthesis gas losses caused by the output of theslag.

Thus, the dual reactor device herein described allows a low residencetime, within a range of approximately 1 to 7 seconds, and the flowregime drags the coal particles through the reactor, operating at a gastemperature of 1,000-1,900° C., which allows to extract the ashes asmelted sludge and avoid the formation of tars and hydrocarbons,minimising the formation of methane, since the production of this latterinvolves an undesired release of energy.

As it will be perfectly known by any person skilled in the art, the sizeof the dual reactor device of the invention will be related and adaptedto the particle size, the reactivity of the fuel, the reactiontemperatures and the speed of the gas phase, and all thesecharacteristics define the time required to allow the occurrence of thegasification reactions. Therefore, the residence time within the reactoris one of the parameters to be guaranteed by means of the describeddesign.

An exemplary embodiment according to the above-mentioned describeddesign allows to obtain the following functional characteristics:

T=approx. 1870° C.

Efficiency=69%.

CO: 60-64% v/v

H₂: 26-27% v/v

CH₄: 0-0.1% v/v

CO₂: 6-7% v/v

Global Reaction:

C+0.38 H₂O+0.36 O₂→0.38 H₂+0.90 CO+0.10 CO₂+0.00 CH₄

In a preferred embodiment of the invention, the temperatures recommendedto carry out processes inside the previously mentioned dual reactordevice are obtained thanks to a high-temperature solar plant and,similarly, the required reagents, steam, oxygen, hydrogen, etc. arederived from a procedure for the obtaining of by-products of theaforementioned solar plant, and preferably, the procedure described inthe Spanish Patent application No. 201031280.

The heat released from the reactor corresponds to an energy portion ofthe heating power of the fuel used, in this case 4,895 Kcal/Kg, which isneither dragged by the gases and slag as chemical energy or sensitiveheat, nor it is used in the reactions. Thus, a first touchstone for theestimation of the heat that must be released through the walls of thereactor by unit of time is the maximum power generated with all theheating power of the fuel used. For that purpose, the walls of thedevice may optionally comprise a heat flow regulating coil, providedthat the flow and the pressure inside the coil are appropriate for thedesign conditions and the materials used, that the pressure inside thecoil is sufficient to maintain the water in its liquid state in all theareas of the dual device, the temperature of each material is within theoperating temperature range of such material and the total finalthickness of the wall has the minimum value. The incorporation of thecoil into the reactor wall is not relevant for the purpose of the globalenergy efficiency of the gasification unit.

1. Coal gasification-pyrolysis dual reactor device, characterised inthat it incorporates the following items: A feeder which feeds wetmilled coal from an upper feeding nozzle (1) split into two branches (1a) and (1 b), which includes a jaw crusher and ball mill inside one ofthe said branches, and a ball mill inside the other branch, to mill theinitial coal material; A screened coal mixing/distribution item (2)located downstream of the feeding and milling nozzles (1), wherein thescreened coal falls inside said mixing/distribution item (2) both as aresult of the steam fed as dragging gas and to the weight of the dustitself, and such circulation of gas fosters the flow of coal particles,minimising their deposition at the walls of the mixing/distribution item(2); A chamber (3), essentially intended for gasification purposes,located downstream of the item (2) where the wet screened material ispartially oxidised through an additional supply of oxygen, at atemperature of approximately 1,800° C.-1,900° C., the interior of thechamber (3) being provided with a refractory cladding, to maintain theinner temperature, as well as an insulating cladding on the outer part,to minimise heat losses caused by radiation to the exterior surface ofthe walls; the chamber (3) is divided into two sub-chambers (3 a, 3 b)by means of a gas diffusion membrane which allows to separate H₂- andcarbon-based gases (CO₂, CO) obtained as a mixture of synthesis gas; acyclone separator (4) located downstream of the chamber (3), whichretains the solid particles present in the synthesis gas, which operatesat a temperature of 600° C. which is maintained by the gas inside theunit; an essentially pyrolytic chamber (5) where the solids carried fromthe cyclone separator (4) are pyrolysed, the free carbon, H₂S and partof CO₂ being essentially eliminated, and the residual gases being fedback into the chamber (3); a sawtooth screen (6) which collects thesolid waste obtained from the simultaneous gasification and pyrolysisprocesses, and which is subsequently stored as slag and ashes in anstorage element for their subsequent reuse.
 2. A coalgasification-pyrolysis dual reactor device according to claim 1,characterised in that 90 mesh (1 a) y 490 mesh (1 b) screens arearranged inside each one of the branches (1 a, 1 b), respectively, ofthe feeding nozzle (1), for the screening of the coal.
 3. A coalgasification-pyrolysis dual reactor device according to claim 1,characterised in that it allows the recirculation of the hydrogenpresent at the chamber (3) from the reactor to the outside, so that itmay be used for other chemical processing operations or to be directlyused as fuel.
 4. A coal gasification-pyrolysis dual reactor deviceaccording to claim 1, characterised in that the operating temperature ofthe item (2) falls within the 220-250° C. range, the relevanttemperature of the chamber (3) is maintained at 1,800° C.-1,900° C. thetemperature of the cyclone separator (4) is 600° C.
 5. A coalgasification-pyrolysis dual reactor device according to claim 1,characterised in that the refractory material of chambers (3) and (5) isbased on an alumina, silica and calcareous fibre cement, preferably ofthe Superboard® type, maintaining the insulation, with dimensionalstability, up to 3,500° C.
 6. A coal gasification-pyrolysis dual reactordevice according to claim 1, characterised in that its walls alsoinclude a coil for the regulation of the heat flow.
 7. A coalgasification-pyrolysis dual reactor device according to claim 1,characterised in that it allows the recirculation of the intermediateresidual gases obtained from one chamber to another, so that the flow ordragged flow regime allows the continuous feeding of coal, and to easilyextract the slag from all the chambers.